Nacreous pigments exhibit pearl-like and/or iridescent effects upon the transmission and reflection of light therethrough. As is well known in the art, the characteristics of such pigments depends upon optical interference phenomena as more fully described, for example, in "The Properties of Nacreous Pigments", Greenstein and Miller, Technical Papers, Vol. XIII, Annual Technical Conference, Society of Plastic Engineers, May 1967.
Nacreous pigments are conventionally formulated for use in suspensions of light transmitting resinous media which can be applied by dipping or spraying operations to form plastic coatings or by extruding, molding, casting or like techniques to provide solid plastic articles incorporating such pigments. Nacreous pigments so utilized should have indexes of refraction which differ from the suspending media because the pearly or nacreous effect displayed by such pigments depends upon the difference between the index of refraction of the oriented, plate-like pigment particles and the index of refraction of the medium in which they are dispersed.
Mica by itself is not a satisfactory nacreous pigment inasmuch as its average index of refraction is about 1.58 which is too close to the index of conventional transparent resinous media of about 1.5-1.59. Excellent nacreous pigments may, however, be provided by the deposition of titanium dioxide or iron oxide coatings on mica flakes.
Linton U.S. Pat. No. 3,087,828 describes the preparation of 20-250.mu. titanium dioxide coated mica nacreous pigments which optionally can be topped with a layer of iron oxide, the latter layer amounting to 0.5 to 20 weight percent of the titanium dioxide. The amount of Fe.sub.2 O.sub.3 is thus 4.9-16.7% (or calculated as Fe is 3.4-11.7%) of the total weight of the two oxides. The patent indicates that the titanium dioxide layer is at least 10 weight percent of the pigment and indicates that 10 weight percent is equivalent to about 50 mg TiO.sub.2 /m.sup.2 mica or a TiO.sub.2 geometric thickness of about 30 m.mu..
Linton U.S. Pat. No. 3,087,829 describes a pigment made by depositing hydrated iron oxide on mica and the coating is then dehydrated and crystallized by means of heat resulting in an iron oxide coated mica pigment. The iron oxide constitutes 10 to 40 weight percent of the pigment.
Brand, et al. U.S. Pat. No. 3,711,308 teaches a coated mica pigment in which a first coat contains titanium or zirconium oxide mixed with iron oxide and a second layer of only the titanium or zirconium oxides which is about twice as thick as the first coating. Bernhard, et al. U.S. Pat. No. 3,874,890 teaches a TiO.sub.2 coated mica pigment which is top coated with iron oxide in an amount up to 20 percent of the final pigment. The TiO.sub.2 coating is about 100-300 mg TiO.sub.2 /m.sup.2 of mica surface which, based on Linton's teachings, means that the geometric thickness of the titanium layer is about 60 m.mu..
Bernhard, et al. U.S. Pat. No. 3,926,659 teaches that in a mica/TiO.sub.2 or ZrO.sub.2 /Fe.sub.2 O.sub.3 pigment, the TiO.sub.2 or ZrO.sub.2 interlayer can be reduced to as little as 5 mg/m.sup.2 if the iron is initially deposited as a single form of iron (III) oxide hydroxide, viz., only one of .alpha., .beta. or .gamma.-FeOOH or magnetite.
Armanini, et al. U.S. Pat. No. 4,146,403 describes iron oxide-coated mica nacreous pigments which are improved by interposing a thin layer of titanium dioxide or aluminum oxide between the iron oxide and the mica. Dark colors and a very good adhesion of the iron oxide layer are obtained.
Pearlescent or nacreous pigments are frequently evaluated by examining or measuring reflectance by means of conventional drawdowns on a hiding power chart. For instance, drawdowns are prepared from a suspension containing 3% pigment in a nitrocellulose lacquer as described, inter alia, in the aforementioned Armanini, et al. patent.
The pearlescent pigments are conventionally used to color various materials. They can be incorporated, for instance, in plastics or coated on a substrate using conventional techniques. In such applications, the pearlescent plastics exhibit a constant pearlescent effect. It has now been surprisingly discovered that certain pearlescent pigments exhibit a unique effect when coated on a color card, a metallic substrate or incorporated into a plastic chip. More particularly, these pigments exhibit unique "color travel", i.e., the apparent color changes as the angle of viewing changes.